U.S. patent application number 13/153920 was filed with the patent office on 2011-09-29 for multi-pole optical signal switch.
Invention is credited to Gil Cohen.
Application Number | 20110234951 13/153920 |
Document ID | / |
Family ID | 44656072 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110234951 |
Kind Code |
A1 |
Cohen; Gil |
September 29, 2011 |
Multi-Pole Optical Signal Switch
Abstract
An optical switch having multiple input and output ports directs
any number of WDM signals, each arriving at a respective input
port, to any one of the output ports. The optical switch includes
an array of LC pixels, each positioned to receive a WDM signal
transmitted through one of the ports, and an array of reflective
elements, each associated with one of the LC pixels. The LC pixels
are controlled to cause a WDM signal incident thereon to attain an
attenuation state while an output of the WDM signal is being
switched by an associated reflective element, such that when an
output for a WDM signal is switched from a first port to a second
port, the switching can be performed in a hitless manner.
Inventors: |
Cohen; Gil; (Livingston,
NJ) |
Family ID: |
44656072 |
Appl. No.: |
13/153920 |
Filed: |
June 6, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12911661 |
Oct 25, 2010 |
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13153920 |
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12066249 |
Sep 3, 2008 |
7822303 |
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PCT/IL2006/001052 |
Sep 10, 2006 |
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12911661 |
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60715695 |
Sep 8, 2005 |
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Current U.S.
Class: |
349/113 |
Current CPC
Class: |
G02B 6/2713 20130101;
H04J 14/0212 20130101; H04Q 2011/0035 20130101; H04Q 11/0005
20130101; H04Q 2011/003 20130101; H04J 14/0205 20130101; G02B
6/3548 20130101; G02B 6/356 20130101; G02B 6/3518 20130101; G02B
6/29313 20130101; G02B 6/29311 20130101; H04J 14/0204 20130101;
H04J 14/0219 20130101; G02B 6/3512 20130101; G02B 6/3594 20130101;
G02B 6/29383 20130101 |
Class at
Publication: |
349/113 |
International
Class: |
G02F 1/1335 20060101
G02F001/1335 |
Claims
1. An optical device comprising: multiple ports for transmitting
wavelength division multiplexing (WDM) signals therethrough; an
array of liquid crystal (LC) pixels, each positioned to receive a
WDM signal transmitted through one of the ports; and an array of
reflective elements, each associated with one of the LC pixels; and
wherein the LC pixels are controlled to cause a WDM signal incident
thereon to attain an attenuation state while an output of the WDM
signal is being switched by an associated reflective element.
2. The device of claim 1, wherein the LC pixels are further
controlled to cause the WDM signal incident thereon to attain a
transmissive state after the output of the WDM signal has been
switched by the associated reflective element.
3. The device of claim 1, wherein the array of reflective elements
comprises an array of independently controlled
micro-electro-mechanical system mirrors.
4. The device of claim 1, wherein the array of reflective elements
are positioned adjacent to the array of LC pixels such that a WDM
signal passes through an LC pixel prior to and after being
reflecting by an associated reflective element.
5. The device of claim 1, wherein the reflective elements are
rotatable about at least one axis.
6. The device of claim 5, wherein the reflective elements are
rotatable about first and second mutually orthogonal axes.
7. The device of claim 1, further comprising: a beam splitting and
combining module provided for each of the ports, the module
including a beam displacing crystal and a half-wave plate.
8. An optical device having multiple ports configured to switch an
output of a wavelength division multiplexing (WDM) signal,
comprising: an array of liquid crystal (LC) pixels, each LC pixel
positioned in an optical path of a WDM signal transmitted through
one of the ports; and an array of reflective elements, each
associated with one of the LC pixels and being movable to multiple
positions to direct a WDM signal transmitted through the associated
LC pixel to any one of the ports, wherein the LC pixels and the
reflective elements are controlled to switch an output of a WDM
signal from a first port to a second port while attenuating
substantially all of the WDM signal as one of the reflective
elements is being moved to switch the output of the WDM signal from
the first port and the second port.
9. The device of claim 8, wherein the array of reflective elements
comprises an array of independently controlled
micro-electro-mechanical system mirrors.
10. The device of claim 8, wherein one of the LC pixels and one of
the reflective elements associated therewith are controlled such
that: (i) a polarization state of a WDM signal incident on the LC
pixel is changed from a first state to a second state, (ii) then
the reflective element is moved from a first position, at which
position the WDM signal is directed to the first port, to a second
position, at which position the WDM signal is directed to the
second port, and (iii) then the polarization state of the WDM
signal incident on the LC pixel is changed from the second state to
the first state.
11. The device of claim 8, wherein the array of reflective elements
are positioned adjacent to the array of LC pixels such that a WDM
signal passes through an LC pixel prior to and after being
reflecting by an associated reflective element.
12. The device of claim 8, wherein the reflective elements are
rotatable about first and second mutually orthogonal axes.
13. The device of claim 8, further comprising: a beam splitting and
combining module provided for each of the ports, the module
including a beam displacing crystal and a half-wave plate.
14. In an optical device having multiple ports including first and
second ports, multiple liquid crystal (LC) pixels including first
and second LC pixels, and multiple reflective elements, a method of
switching outputs of multi-pole optical beams, each of which is
supplied through a different one of the ports, comprising:
controlling a first LC pixel to change a polarization state of a
multi-pole optical beam incident thereon from a first state to a
second state; after the polarization state of the multi-pole
optical beam has been changed from the first state to the second
state, moving a reflective element associated with the first LC
pixel from a first position, at which position the multi-pole
optical beam is directed to the first port, to a second position,
at which position the multi-pole optical beam is directed to the
second port; and after the reflective element associated with the
first LC pixel has been moved from the first position to the second
position, controlling the first LC pixel to change the polarization
state of the multi-pole optical beam from the second state to the
first state.
15. The method of claim 14, further comprising: controlling a
second LC pixel on which a different multi-pole optical beam is
incident to change a polarization state of the different multi-pole
optical beam from the first state to the second state; after the
polarization state of the different multi-pole optical beam has
been changed from the first state to the second state, moving a
reflective element associated with the second LC pixel from a first
position, at which position the different multi-pole optical beam
is directed to a third port, to a second position, at which
position the different multi-pole optical beam is directed to
fourth port; and after the reflective element associated with the
second LC pixel has been moved from the first position to the
second position, controlling the second LC pixel to change the
polarization state of the different multi-pole optical beam from
the second state to the first state.
16. The method of claim 15, wherein the reflective elements are
moved by rotating the reflective element about first and second
mutually orthogonal axes.
17. The method of claim 15, wherein the LC pixels and the
reflective elements associated therewith are closely positioned
relative to each other so that the multi-pole optical beam is
directed through the first LC pixel twice and the different
multi-pole optical beam is directed through the second LC pixel
twice.
18. The method of claim 15, wherein the multi-pole optical beam
supplied through one of the ports and incident on the second LC
pixel has the same spectral composition as the multi-pole optical
beam output through the fourth port.
19. The method of claim 14, wherein the multi-pole optical beam
supplied through one of the ports and incident on the first LC
pixel has the same spectral composition as the multi-pole optical
beam output through the second port.
20. The method of claim 14, wherein, while moving the reflective
element associated with the first LC pixel from the first position
to the second position, the multi-pole optical beam having the
second polarization state is scanned from the first port to the
second port in accordance with a movement of the reflective
element.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/911,661, filed Oct. 25, 2010, which is a
continuation of U.S. patent application Ser. No. 12/066,249, filed
Sep. 3, 2008, now U.S. Pat. No. 7,822,303, which is a national
stage application of PCT Application No. PCT/IL2006/001052, filed
Sep. 10, 2006, which claims the benefit of U.S. Provisional Patent
Application Ser. No. 60/715,695, filed Sep. 8, 2005.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] One or more embodiments of the present invention relate to
the field of optical communication networks and in particular to an
optical router for directing any input optical signal arriving at
any one or more input ports in a network node to one or more output
ports.
[0004] 2. Description of the Related Art
[0005] It is known in the field of optical communications to use
optical wavelengths as optical carriers for carrying digital or
analog information. Also, the different wavelengths may be used to
discriminate one set or channel of information from another. When a
plurality of wavelengths are coupled or multiplexed onto a single
fiber, this is called wavelength division multiplexing (WDM). Use
of such WDM increases the overall bandwidth of the system.
[0006] There is a need in such systems to switch packets of optical
information passing along one fiber to any of a number of other
fibers, according to the wavelength of the optical signal. Such a
switch is known as an optical router or a wavelength selective
switch. A number of wavelength dependent switches and routers exist
in the prior art. In co-pending PCT Applications. PCT/IL2002/00511,
PCT/IL2003/01002 and PCT/IL2006/00590, all hereby incorporated by
reference, each in its entirety, there are disclosed wavelength
selective switches wherein an input optical signal is spatially
wavelength-dispersed and polarization-split in two preferably
perpendicular planes. The wavelength dispersion is preferably
performed by a diffraction grating, and the polarization-splitting
by a polarized beam splitter. A polarization rotation device, such
as a liquid crystal polarization modulator, pixilated along the
wavelength dispersive direction such that each pixel operates on a
separate wavelength channel, is operative to rotate the
polarization of the light signal passing through each pixel,
according to the control voltage applied to the pixel. The
polarization modulated signals are then wavelength-recombined and
polarization-recombined by means of similar dispersion and
polarization combining components as were used to respectively
disperse and split the input signals. At the output polarization
recombiner, the direction in which the resulting output signal is
directed is determined by whether the polarization of the
particular wavelength channel was rotated by the polarization
modulator pixel, or not. PCT Application Nos. PCT/IL2003/01002 and
PCT/IL2006/00590 also incorporate lateral expansion of the
polarized beams in the plane of the dispersion.
[0007] Such fast, wavelength selective, optical switch structures
are capable of use in WDM switching applications, but are generally
limited to 2.times.2 configurations, for use as channel blockers or
attenuators. In U.S. Pat. No. 7,092,599 to S. J. Frisken for
"Wavelength Manipulation System and Method," there is described a
wavelength manipulation system using an LCOS phased array, with an
optical arrangement including a spherical mirror and a cylindrical
lens for maintaining collimation of the input beams in the
direction of dispersion, and for focusing of the input beams in the
direction perpendicular to the direction of dispersion. In
published U.S. Patent Application No. 2006/0067611 for "Wavelength
Selective Reconfigurable Optical Cross Connect," there is described
an optical coupling device using art LCOS phased array, with an
optical arrangement including at least a cylindrical mirror and a
cylindrical lens.
[0008] There therefore exists a need for a new optical, multi-pole,
multi-way wavelength selective switch structure having a simple
optical structure, for use in channel routing applications, with
the addition of add and drop functionalities. In addition, there
exists a need for a new optical, multi-pole, multi-way switch
structure that can route WDM signals from any input port to any
output port in a hitless manner.
[0009] The disclosures of each of the publications mentioned in
this section and in other sections of the specification are hereby
incorporated by reference, each in its entirety.
SUMMARY OF THE INVENTION
[0010] An embodiment of the present invention seeks to provide a
new fiber-optical, multi-way, wavelength selective switch (WSS)
structure, such as is used for channel routing and/or blocking
applications in optical communication and information transmission
systems. Add and drop functionality, from and to a number of ports,
can also be implemented in this switch structure. The switch uses a
minimum of components, and can thus be economically constructed for
large scale use in such systems. The switch structure can also be
used as a wavelength selective variable optical attenuator for any
of the transfer routes therethrough.
[0011] The switch structure utilizes conversion, preferably by the
use of birefringent crystals, of optical signals input to any port
of the switch, to light beams having a defined polarization,
preferably linear, and which are mutually disposed in a
predetermined plane with respect to the system plane in which
optical manipulation of the beam traversing the WSS is to be
performed. This is followed by lateral expansion of the polarized
beams in this predefined plane. This lateral expansion is
preferably performed by means of a pair of anomorphic prisms,
though any alternative method can be utilized, such as a
cylindrical lens telescope system, or even a single prism, as is
known in the art. The beam is then spatially wavelength-dispersed
in the same predetermined plane as that of the beam expansion,
preferably by means of a diffraction grating. Lateral expansion of
the beam, combined with dispersion in the same plane as that of the
lateral expansion provides the WSS with advantages compared to
prior art switches, especially with respect to the reduction in
switch height thus enabled, and with respect to the increased
wavelength resolution thus enabled. The light is then directed
through a polarization rotation device, preferably a liquid crystal
(LC) cell pixilated along the wavelength dispersive direction, such
that each pixel operates on a separate wavelength. When the
appropriate control voltage is applied to a pixel, the polarization
of the light signal passing through that pixel is rotated, thereby
blocking, transmitting or attenuating the particular wavelength
channel passing through that pixel.
[0012] After beam polarization rotation, the light passing through
each pixel is angularly deflected using a beam steering element.
The beam steering element is pixilated along the wavelength
dispersive direction, such that each beam steering pixel also
operates on a separate wavelength. When the appropriate control
voltage is applied to a beam steering pixel, the wavelength
component associated with that liquid crystal pixel is steered by
the beam steering pixel towards its desired direction. The beam can
be steered either in the plane of the wavelength dispersion,
conventionally called the horizontal direction of the switch
structure, or perpendicular thereto, known as the vertical
direction of the switch structure. Perpendicular steering has an
advantage in that the angular deviation generated by the beam
steering does not interfere with the angular deviation generated by
the wavelength dispersion, thus simplifying construction.
[0013] The steering of the beam through each individual pixel
enables light of different wavelengths, after being transmitted or
attenuated, to be directed to different output ports, according to
the various paths defined by the beam steering angles.
Additionally, the light of a specific wavelength can be blocked, in
which case the beam steering is unused.
[0014] The wavelength dispersed, steered beams from the
polarization rotation pixels are then recombined, followed by beam
contraction and passage back through an output birefringent crystal
towards the switch outputs. The wavelength selective switch can be
reflective, in which case the steered beams are returned through
the same device as was used to disperse the multiwavelength input
beams, through the same lateral beam expander that was used to
laterally expand the input beams, and through the same polarization
manipulator that was used on the input beams. Because of the beam
steering of the present invention, each angularly displaced,
steered beam passes through these components at a slightly
displaced location, depending on the beam steering angle, and an
array of output collimators is disposed at the end of the output
birefringent crystal to collect each steered beam at a separate
output port according to the steered beam angle. Alternatively and
preferably, the WSS can be transmissive, in which case the steered
beams are output from the device through separate dispersive
elements, beam contracting elements and polarization manipulation
elements.
[0015] The WSS of the present invention has a significant advantage
over prior art switches, in that the polarization rotation element
can be operated in co-operation with the beam steering device in
such a manner that the steered beam is prevented from coupling into
any output ports other than its destined output port. This is
achieved by adjusting the transmissivity of the polarization
rotation device in the pathways to undesired output ports, such
that output to them is blocked as the steered beam passes over
them. In this way, a hitless switching configuration can be
achieved using only a one-dimensional steering array.
[0016] The beam steering elements can be any miniature element
which is capable of deviating the path of the beam impinging
thereon. According to one preferred embodiment, an array of
Micro-Electro-Mechanical System (MEMS) components, such as
micro-mirrors, are used to generate the steering. The angle of
deviation of such MEMS elements can be controlled electronically to
provide the desired beam steered angle.
[0017] Alternatively and preferably, the beam steering can be
performed by utilizing a set of serially disposed liquid crystal
arrays and prismatic polarization separators, such as wedge shaped
birefringent walk-off crystals, which generate different angles of
propagation to the beam passing therethrough, according to the
different polarizations of the beams produced by the setting of the
liquid crystal array pixels. The steered angle of a beam passing
through a particular liquid crystal pixel is determined by the
polarization rotation setting of each of the serial LC pixels
through which the beam passes. This embodiment has the advantage of
generating the beam steering without any moving parts, but the
disadvantage of a more complex control system and possibly higher
cross-talk between channels.
[0018] According to a further preferred embodiment, the beam
steering can be generated by use of a liquid crystal-on-silicon
(LCOS) spatial light modulator acting as a phased array. In an LCOS
device, the light is passed through a pixilated layer of liquid
crystal material layer disposed over a reflective substrate formed
on the front of a CMOS substrate, on which is implemented a circuit
array for driving the various pixels of the LC layer between their
states. According to these states, the light traverses each pixel
either undeflected, and is reflected back along its incident path,
or is deflected and thus reflected back along a different path to a
different optical port from that by which it reached the LCOS
pixel. The pixels on such a device are generally so small that each
wavelength component covers a number of pixels, even with the
smallest optical dimensioning practical. The complete 2-dimensional
LCOS array is then programmed to direct the various wavelength
components of the input channels to the directions desired for each
wavelength according to the phase shifts applied to the various
pixels in the LC pixels for each wavelength.
[0019] In all of the preferred WSS embodiments of the present
invention, the operation of the device is essentially reciprocal,
such that signals for switching can be input at what has been
described in this application as "output" ports, and conversely,
can be output at what has been called "input" ports. It is
therefore to be understood that the terms input and output in this
application can be used interchangeably, and are also claimed in
such manner, and that the invention is not intended to be limited
by the directional nomenclature of a particular port. Wherever a
particular port is to be used for input or output, it is to be
understood that a signal separation device such as a circulator has
to be used to separate the input from the output directional
signals.
[0020] The channel switching rate is determined by the slower of
the switching rates of both the LC
blocking/transmitting/attenuating element, and of the beam steering
device, since the beam must be processed by both. In either the
case of MEMS or of LC beam steering, the rate achievable is
suitable for use in WDM or DWDM switching applications.
[0021] There is therefore provided, according to a first preferred
embodiment of the present invention, a wavelength selective switch
(WSS) comprising: (i) at least a first port for inputting at least
a first multi-wavelength optical signal, (ii) a plurality of output
ports for outputting different wavelength components of the at
least first multi-wavelength optical signal, (iii) a polarization
transformation device for converting each of the at least first
multi-wavelength optical signals into a pair of multi-wavelength
optical beams disposed in a predetermined plane and having the same
predefined polarization, (iv) a beam expanding device for laterally
expanding the multi-wavelength optical beams of predefined
polarizations in the predetermined plane, (v) a wavelength
dispersive element receiving the laterally expanded optical beams
of predefined polarizations and dispersing wavelength components
thereof in the predetermined plane, (vi) a polarization rotation
element, pixilated generally along the direction of the dispersion,
adapted to rotate the polarization of light passing through pixels
thereof according to control signals applied to the pixels, such
that the polarization of at least one wavelength component of the
dispersed optical beams is rotated according to the control signal
applied to the pixel through which the at least one wavelength
component passes, and (vii) a pixilated beam steering element
disposed such that the at least one wavelength component passing
through a pixel of the polarization element is steered towards its
desired output port according to the settings of the pixel of the
beam steering device associated with the at least one wavelength
component.
[0022] In the above described WSS, the at least one wavelength
component is preferably attenuated in accordance with the control
signal applied to the pixel of the polarization rotation element
associated with the at least one wavelength component. The beam
steering element may be any of an array of Micro Electro-Mechanical
System (MEMS) mirrors, each mirror of the array having a single
axis of rotation, or a Liquid Crystal on Silicon (LCOS) array, or a
sequence of pairs of adjustable polarization rotation elements and
birefringent prisms, wherein the at least one wavelength component
is steered in accordance with the settings of the adjustable
polarization rotation elements through which the at least one
wavelength component passes.
[0023] In accordance with still another preferred embodiment of the
present invention, any of the above described switches may
preferably further comprise at least one optical element for
focusing the dispersed wavelength components of the expanded light
beams onto the beam steering element. This focusing may be
performed by a lens, or by use of a wavelength dispersive element
also having optical focusing power.
[0024] Additionally, in the above described WSS's, the polarization
rotation element may be a liquid crystal element; the polarization
transformation device may be a birefringent walk-off crystal with a
half-waveplate disposed on part of its output face; the beam
expanding device may be any one of a pair of anomorphic prisms, a
cylindrical lens telescope system, and a single prism; and the
wavelength dispersive element may be a diffraction grating.
[0025] In the above-described embodiments using a MEMS array, the
pixel of the polarization rotation element associated with the at
least one wavelength component may be preferably controlled to
block the passage of the at least one wavelength component during
switching, at least when the at least one wavelength component
crosses a path to an undesired output port.
[0026] According to a further preferred embodiment, the pixilated
beam steering element, of whatever type, is adapted to steer the at
least one wavelength component in a direction such that the steered
wavelength component does not cross the path of any other undesired
wavelength component.
[0027] There is further provided in accordance with still another
preferred embodiment of the present invention, a WSS as described
above, and also comprising a beam demagnifier disposed such that
the dimensions of the multi-wavelength optical beams are reduced in
the direction perpendicular to the plane of dispersion.
[0028] Furthermore, in any of the above-described embodiments, the
pixilated beam steering element may either be a reflective element,
such that the steered beam accesses its destined output port
through those optical components used to direct the optical signal
from the input port to the beam steering element, or it may be a
transmissive element, such that the steered beam accesses its
destined output port through additional optical components which
direct the optical signal from the beam steering element to the
output port.
[0029] Additionally, any of the above-described embodiments may
further comprise a beam monitoring array for determining the signal
level in any port.
[0030] In accordance with still another preferred embodiment of the
present invention, there is further provided a wavelength selective
switch comprising: (i) at least a first port for inputting at least
a first multi-wavelength optical signal, (ii) a plurality of output
ports for outputting different wavelength components of the at
least first multi-wavelength optical signal, (iii) a polarization
transformation device for converting each of the at least first
multi-wavelength optical signals into a pair of multi-wavelength
optical beams disposed in a predetermined plane and having the same
predefined polarization, (iv) a beam expanding device for laterally
expanding the multi-wavelength optical beams of predefined
polarizations in the predetermined plane, (v) a wavelength
dispersive element receiving the laterally expanded optical beams
of predefined polarizations and dispersing wavelength components
thereof in the predetermined plane, and (vi) a beam steering
element comprising a pixilated Liquid Crystal on Silicon (LCOS)
array, the LCOS array being configured to direct different ones of
the wavelength components to output ports in accordance with
control signals applied thereto. The pixilated LCOS array may
preferably be configured to attenuate different ones of the
wavelength components in accordance with the control signals
applied thereto.
[0031] Any of the above described WSS's incorporating an LCOS
array, preferably further comprises a beam deflecting element
adapted to increase the steered beam deflection angle. This beam
deflecting element may be any one of a diffractive optical element,
a holographic element, a sequential series of reflecting surfaces,
and a divergent prism assembly.
[0032] There is further provided in accordance with still another
preferred embodiment of the present invention, a wavelength
selective switch comprising: (i) at least a first port for
inputting a multi-wavelength optical signal, (ii) a plurality of
output ports for outputting different wavelength components of the
multi-wavelength optical signal, (iii) a beam expanding device for
laterally expanding at least one beam generated from the
multi-wavelength optical beams in a predetermined plane, (iv) a
wavelength dispersive element receiving the at least one laterally
expanded optical beam and dispersing wavelength components thereof
in the predetermined plane, (v) a pixilated beam attenuating array
operating on the dispersed wavelength components, and (vi) a
pixilated beam steering element adapted to steer at least one of
the dispersed wavelength components towards a desired output port,
wherein the pixilated beam attenuating array is operated in
co-operation with the beam steering device in such a manner that
the steered beam is prevented from coupling into any output ports
other than its desired output port.
[0033] In such a WSS, the pixilated beam attenuating array may
preferably be controlled to block transmission of the steered beam
during switching, at least while it traverses the paths to output
ports other than the desired output port.
[0034] In accordance with still another preferred embodiment of the
present invention, there is further provided a method of switching
selected wavelength components of a multi-wavelength input optical
signal to a desired output port, the method comprising the steps
of: (i) generating at least one beam from the multi-wavelength
input optical signal, (ii) laterally expanding the at least one
multi-wavelength optical beam in a predetermined plane, (iii)
spatially dispersing in the predetermined plane the at least one
multi-wavelength optical beam to generate wavelength components
thereof, (iv) providing a pixilated beam attenuating array to
attenuate the dispersed wavelength components, and (v) steering at
least one of the dispersed wavelength components towards a desired
output port, wherein the steering is performed in co-operation with
the attenuation in such a manner that the steered beam is prevented
from coupling into any output ports other than its desired output
port.
[0035] According to this method, the pixilated beam attenuating
array may preferably be controlled to block transmission of the at
least one steered wavelength component during switching, at least
while it traverses the paths to output ports other than the desired
output port.
[0036] In accordance with a still further preferred embodiment of
the present invention, there is also provided a method of switching
selected wavelength components of at least one multi-wavelength
input optical signal to a desired output port, the method
comprising the steps of: (i) transforming the polarization of each
of the at least one multi-wavelength optical signals into a pair of
multi-wavelength optical beams having predefined polarizations,
(ii) laterally expanding the multi-wavelength optical beams of
predefined polarizations in a predetermined plane, (iii) spatially
dispersing in the predetermined plane, the laterally expanded,
multi-wavelength optical beams into a series of spatially separated
wavelength beams, (iv) utilizing a polarization rotation element,
pixilated generally along the direction of the dispersion, for
rotating the polarization of light passing through pixels thereof
according to control signals applied to the pixels, such that the
polarization of at least one wavelength component of the dispersed
optical beams is rotated according to the control signal applied to
the pixel through which the at least one wavelength component
passes, and (v) steering the at least one wavelength component
passing through a pixel of the polarization element, by use of a
pixilated beam steering device, towards its desired output port
according to the settings of the pixel associated with the at least
one wavelength component, of the beam steering device.
[0037] There is further provided in accordance with yet more
preferred embodiments of the present invention, the above described
method, modified by the incorporation of any of the adaptations,
additions or limitations described in relation to the WSS
embodiments described immediately hereinabove.
[0038] Further embodiments of the present invention provide an
optical switch having multiple input and output ports, that can
direct any number of WDM signals, each arriving at a respective
input port, to any one of the output ports. According to such
embodiments of the present invention, when an output for a WDM
signal is switched from a first port to a second port, the
switching can be performed in a hitless manner.
[0039] An optical device, according to an embodiment of the present
invention, includes multiple ports for transmitting WDM signals
therethrough, an array of LC pixels, each positioned to receive a
WDM signal transmitted through one of the ports, and an array of
reflective elements, each associated with one of the LC pixels. The
LC pixels are controlled to cause a WDM signal incident thereon to
attain an attenuation state while an output of the WDM signal is
being switched by an associated reflective element. A control unit
may be provided for controlling the LC pixels and the reflective
elements and to synchronize the timing of the control of the LC
pixels relative to the switching of the associated reflective
elements.
[0040] An optical device, according to another embodiment of the
present invention, includes multiple ports, an array of LC pixels,
each LC pixel positioned in an optical path of a WDM signal
transmitted through one of ports, and an array of reflective
elements, each associated with one of the LC pixels and being
movable to multiple positions to direct a WDM signal transmitted
through the associated LC pixel to any one of the ports. The LC
pixels and the reflective elements are controlled to switch an
output of a WDM signal from a first port to a second port while
attenuating substantially all of the WDM signal as one of the
reflective elements is being moved to switch the output of the WDM
signal from the first port and the second port. A control unit may
be provided for controlling the LC pixels and the reflective
elements and to synchronize the timing of the control of the LC
pixels relative to the switching of the associated reflective
elements.
[0041] A method of switching outputs of multi-pole optical beams in
an optical device having multiple ports, LC pixels and reflective
elements, according to an embodiment of the present invention,
includes the steps of: (i) controlling a first LC pixel to change a
polarization state of a multi-pole optical beam incident thereon
from a first state to a second state, (ii) after the polarization
state of the multi-pole optical beam has been changed from the
first state to the second state, moving a reflective element
associated with the first LC pixel from a first position, at which
position the multi-pole optical beam is directed to the first port,
to a second position, at which position the multi-pole optical beam
is directed to the second port, and (iii) after the reflective
element associated with the first LC pixel has been moved from the
first position to the second position, controlling the first LC
pixel to change the polarization state of the multi-pole optical
beam from the second state to the first state.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] The present invention will be understood and appreciated
more fully from the following detailed description, taken in
conjunction with the drawings.
[0043] FIG. 1 illustrates schematically a block diagram of the
functionality of an optical wavelength router according to a first
preferred embodiment of the present invention.
[0044] FIG. 2 illustrates schematically the structure of a
reflective embodiment of FIG. 1, using beam steering.
[0045] FIG. 3 is a schematic plan view, showing approximate
component location and layout, of a reflective wavelength selective
router, constructed and operative according to another preferred
embodiment of the present invention.
[0046] FIGS. 4A and 4B are more schematic views of the reflective
wavelength selective router of FIG. 3, showing the component parts
in more detail.
[0047] FIGS. 5A and 5B are schematic views from top and side of a
MEMS based beam steering focal plane module for use in the router
of FIG. 3; FIGS. 5C to 5E illustrate schematically another
preferred embodiment of the present invention, providing a hitless
beam steering configuration.
[0048] FIGS. 6A and 6B are schematic views from top and side of a
liquid crystal/birefringent prism based beam steering focal plane
module for use in the router of FIG. 3.
[0049] FIGS. 7A and 7B are schematic views of two transmissive beam
steering modules, similar to those illustrated in FIG. 6B, but
showing different wedge dispositions.
[0050] FIGS. 8A and 8B are schematic illustrations from side and
front, of the fiber interface module of the router according to a
preferred embodiment of the present invention.
[0051] FIG. 9 is a schematic side view of the fiber interface input
module shown in FIG. 8A, but including an inverse telescope for
demagnifying the height of the array of input beams.
[0052] FIG. 10 is a schematic side view of an alternative
configuration for the location of the inverse telescope of the
fiber interface input module of FIG. 9.
[0053] FIGS. 11A and 11B illustrate a schematic wavelength
selective Add/Drop router module, constructed and operative
according to a further preferred embodiment of the present
invention, using beam steering.
[0054] FIG. 12 shows the different steering angles generated by a
MEMS mirror, directing the beam between the input/output port and
any of the other ports of the router of FIGS. 11A and 11B.
[0055] FIG. 13 illustrates a further preferred embodiment of the
present invention, in which MEMS devices, based on reflection from
mirrors, are used in a transmissive embodiment of the router of the
present invention.
[0056] FIG. 14 illustrates yet a further preferred embodiment of
the beam steering module for use in the reflective embodiments of
the present invention using a phased array liquid
crystal-on-silicon (LCOS) device.
[0057] FIG. 15 is a schematic plan view of a multi-pole optical
signal switch according to one or more embodiments of the present
invention.
[0058] FIG. 16 is a schematic isometric view of the multi-pole
optical signal switch shown in FIG. 15.
[0059] FIGS. 17A and 17B schematically illustrate different
implementations of the multi-pole optical signal switch of FIGS. 15
and 16, in which the input and output fiber ports are aligned in
one contiguous array.
[0060] FIG. 18A schematically illustrates another implementation of
the multi-pole optical signal switches using two one-dimensional
MEMS mirror arrays where output ports are arranged in a
one-dimensional array.
[0061] FIG. 18B schematically illustrates another implementation of
the multi-pole optical signal switches using two one-dimensional
MEMS mirror arrays where output ports are arranged in a
two-dimensional array.
[0062] FIG. 19 shows an end view of a polarization selection module
of FIGS. 15 and 16, with its half-wave plate covering one part of
the output port.
[0063] FIG. 20 illustrates schematically a hitless beam steering
configuration according to one or more embodiments of the present
invention.
DETAILED DESCRIPTION
[0064] Reference is now made to FIG. 1, which illustrates
schematically a block diagram of the functionality of an optical
wavelength router, including a single input port, a single main
output port and a number of Drop ports. The function of the router
is to either transmit, to block or to attenuate any wavelength
channel in the input signal, and to direct that signal, if
transmitted or attenuated, to any of the output or Drop ports.
[0065] Reference is now made to FIG. 2, which illustrates
schematically the router 20 of FIG. 1, as implemented according to
a preferred embodiment of the present invention, in the form of a
block diagram of the functionality of the separate operative parts
of the router. The signals are input to the router through a Fiber
Interface Block 21, which accepts the input signals and converts
them into free-space beams for polarization processing, lateral
expansion and spatial manipulation. According to a preferred
embodiment of the present invention, the free-space beams are first
polarization processed in the polarization selection module 22, to
generate pairs of beams mutually disposed in a predetermined plane,
with like polarization, and then spatially expanded in that plane
by means of a beam expander block, 23, by one of the methods known
in the art. The spatially expanded beams are then dispersed in that
plane in a Dispersion Optics Module, 24. Finally, the dispersed
beam wavelength components are directed onto a Focal Plane Beam
Steering Module 25, which incorporates a pixilated LC array for
selecting the optical transmissibility applied to each wavelength
channel, and a pixilated beam steering array which directs each
wavelength channel in the selected direction according to which
output or Drop port is to be selected for each wavelength channel.
The dispersed beam wavelength components are preferably directed
onto the Focal Plane Beam Steering Module 25, by an element with
positive optical power, which can either be a separate focusing
element such as a lens, or can be implemented by use of a
dispersion element also having optical focusing power.
[0066] FIG. 2 illustrates schematically the structure of a
reflective embodiment of FIG. 1, in that after transmission
processing and beam steering, each wavelength channel beam is
returned by reflection in a reflective surface incorporated into
the beam steering module 24, back through the Dispersion Optics
Module 22 to the beam expander block 22, and to the Fiber Interface
Block 21, which is operative, in addition to its input role, also
to output the switched beams to their selected output fibers. Such
a reflective arrangement provides the most cost-effective and
compact embodiment of this invention. It is to be understood,
however, that a transmissive embodiment based on the component
parts of FIG. 2 is equally feasible, with the Dispersion Optics
Module 23, the beam expander block 22, and the Fiber Interface
Block 21 repeated in that order after the Beam Steering Module 24,
i.e., to the right of it in FIG. 2. Such a transmissive embodiment
is understood to be included in all of the generalized embodiments
of the present invention, where the exact optical arrangement,
i.e., transmissive or reflective, is not specified. Detailed
descriptions of some reflective and transmissive embodiments are
given hereinbelow.
[0067] Reference is now made to FIG. 3, which is a schematic plan
view, showing preferred component location and layout of a
reflective wavelength selective routing switch, constructed and
operative according to another preferred embodiment of the present
invention. The embodiment of FIG. 3 shows a fiber interface
input/output block, which includes a polarization conversion device
30, such as a birefringent walk-off crystal with a half wave plate
over part of its output. The like-polarization free-space beams
thus generated are passed to a one dimensional beam expander 31,
shown in this embodiment as a pair of anomorphic prisms, which are
operative to expand the beams in the plane of the drawing. The
expanded beams are then directed to a dispersive grating element
32, shown in this embodiment as a reflective grating, which
disperses the wavelength components of each input beam in the same
plane as that in which the beams were generated and expanded,
namely, in the plane of the drawing paper, and a focusing lens 33
focuses the separated wavelength components onto the focal plane
beam steering module 34, which is shown in more detail in the
following drawings. It is to be understood that the wavelength
selective router can equally be implemented in a transmissive
embodiment, as explained hereinabove.
[0068] Reference is now made to FIG. 4A, which is another schematic
plan view of the reflective wavelength selective router of FIG. 3,
showing the component parts in more detail. FIG. 4A shows the plan
view layout of a single channel path of the router. The input (or
output) beam of each port is input (or output) at the fiber
interface block, which preferably comprises a fiber collimator 40
per port, followed by a birefringent walk-off crystal 41, such as a
YVO.sub.4 crystal, preferably having a half wave plate 42 over part
of its output face. The output of each channel thus comprises a
pair of beams having the same polarization direction, as indicated
by the vertical line on each of the beam outputs, and disposed in a
predetermined plane, which, in the example shown in FIG. 4A, is in
the plane of the drawing. After this polarization decomposition and
conversion, these beams are then laterally expanded in that same
predetermined plane, in the preferred example shown in FIG. 4A, by
an anomorphic prism pair 43. These laterally expanded beams are
passed to the grating 44 for wavelength dispersion, again in the
same predetermined plane, which, in the example shown in FIG. 4A,
is in the plane of the drawing. The dispersed wavelength components
are then directed to the lens 45 for focusing on the beam switching
and steering module 46. The beams of each wavelength channel are
first switched by the pixilated liquid crystal (LC) array 47, to
achieve the desired transmission state for that channel, either a
blocked, or a fully transmitted or an attenuated transmission
state. After the appropriate beam processing by the LC array, the
beam is then passed to the beam steering device 48, shown in FIG.
4A as a reflective element, operative to reflect each switched and
steered beam back down the router to the output positions of the
birefringent crystal, and from there to the respective output
collimator ports. This steering is performed in the direction
perpendicular to the plane of the drawing. According to one
preferred embodiment, the beam steering device may be a MEMS array
of mirrors. The birefringent walk-off crystal 41 with its half-wave
plate 42, is shown on a larger scale and in end view in FIG. 4B,
where the beam positions can be seen after decomposition of each
input beam into the two polarization-defined side-by-side beams 49.
In the embodiment shown in FIGS. 4A and 4B, the beam steering is
performed out of the plane of the drawing, hence the vertical line
of beams seen in FIG. 4B, one pair for each channel. As previously
stated, a similar transmissive embodiment can equally be
implemented, in which case the reflective elements 48 are replaced
by a transmissive steering element embodiment, with the above
mentioned input elements of the device repeated to the right of the
beam steering device to deal with the outputting of the transmitted
beams.
[0069] Reference is now made to FIGS. 5A and 5B, which are
schematic views respectively from the top and side of a preferred
embodiment of the beam steering focal plane module of the previous
drawings, in the form of a MEMS array, for use with a single input
channel of a WSS of the present invention. In FIG. 5A there are
seen (i) the LC polarization rotation array 50, pixilated in the
direction 54 of the wavelength dispersion, and responsible for
selecting the desired transmissive, blocked or attenuated state of
each wavelength channel, (ii) an optional linear polarizing element
51, whose function is to increase the extinction ratio of the
polarization selection combination in the system, and thus to
improve the blocking, and (iii) a one-dimensional MEMS array of
mirrors 52, each of the MEMS mirrors being aligned directly behind
a corresponding pixel of the LC array. The MEMS array mirrors
reflectively steer the beam from each pixel back through that
pixel, but at a steered angle out of the plane of the drawing, each
pixel according to the setting of the MEMS control 53 for each
mirror of the array.
[0070] FIG. 5B shows the same arrangement as that of FIG. 5A, but
from a side view, i.e., looking along the dispersion direction, so
that the different steering angles to the differently labeled
output ports 1 to 3 can be shown. In the example shown in FIG. 5B,
the input port, labeled I/O, can also be utilized as an output port
by incorporation of a circulator at the input/output port of the
router, as is known in the art. The additional three outputs shown
can be used as Drop ports, though functionally, since they operate
no differently from the input/output port shown, this is merely a
matter of nomenclature, and the device is essentially
reciprocal.
[0071] In the MEMS configuration illustrated in FIGS. 5A and 5B, a
problem may arise during beam steering from one output to another
because the MEMS element sweeps the beam into positions providing
outputs where they are not requested. Thus for example, in FIG. 5B,
when the beam of any particular wavelength is steered between
outputs 1 and 3, it will momentarily cross output 2, generating a
spurious signal therein.
[0072] Reference is now made to FIGS. 5C to 5E which illustrate
schematically another preferred embodiment of the present
invention, which avoids this phenomenon, and provides what is known
as a hitless beam steering configuration.
[0073] Referring first to FIG. 5C, which illustrates prior art
switching methods, switching the beam from the port labeled
input/output to output 3 involves traverse of output ports 1 and 2,
with resulting spurious signals thereto. Furthermore, in such
systems which utilize only single-direction beam steering, without
the polarization rotation attenuation effects of the present
invention, attenuation of the transmitted beam is achieved by
directing the transmitted beam 55, so that it does not completely
overlap the destination port 56, thereby coupling in only part of
the signal. This, however, has the drawback that the band pass
shape of the beam changes with the attenuation level.
[0074] Reference is now made to FIG. 5D, which illustrates a method
by which prior art switches can overcome the problem of spurious
signals. By using a two dimensional beam steering device, such as a
MEMS mirror array having two axes of rotation for each mirror
pixel, the beam can be deflected though a path 57 such that it will
not illuminate in the direction of any other output port before
reaching its target port 56. However, such a twin steered axis MEMS
array is more costly to manufacture and incorporate, and more
difficult to control, than a single steered axis MEMS array.
Furthermore, when no polarization rotation attenuation is used, the
same disadvantage arises as was described in connection with the
embodiment of FIG. 5C.
[0075] Reference is now made to FIG. 5E, which illustrates a method
by which the beam steering configuration of the WSS of the present
invention, is able to overcome both of these drawbacks of prior art
methods, and without forgoing the use of a simple one dimensional
MEMS mirror array with single axis steering. In the configuration
of FIG. 5E, the switched beam is steered directly between the input
port and the destination port 56, but while the beam is passing
over the intermediate ports during the switching process, the beam
transmission is blocked by controlling the settings of the LC
polarization rotation pixels associated with the particular
wavelength component being switched. As soon as the switching
process is over, and the beam path connection to the desired
destination port is completed, the transmission can be unblocked
and the switch can operate as programmed. The blocked paths to the
undesired ports are shown schematically in FIG. 5E by the blocking
patch 58. By this means, the problem of spurious signals can be
overcome.
[0076] Furthermore, use of the polarization rotation attenuating
elements of the present invention, allows the switched beam 55 to
couple completely into its destination port 56, and any desired
attenuation can be achieved by adjustment of the LC pixel setting
to control the channel attenuation directly. In this way, the band
pass shape distortion associated with the switching schemes of FIG.
5C and FIG. 5D is also avoided.
[0077] Reference is now made to FIGS. 6A and 6B, which are
schematic views respectively from the top and side of an
LC/birefringent-prism based beam steering focal plane module,
constructed and operative according to a further preferred
embodiment of the present invention. For illustrative purposes
only, the FIG. 6A embodiment is shown as a reflective
configuration, while that shown in FIG. 6B is transmissive, though
it is to be understood that either configuration may be used as a
reflective or transmissive embodiment. In FIG. 6A, there is shown
the focusing lens 60 of the router directing the beams towards the
focal plane switching and steering array. The switching function
itself, namely the decision as to whether a particular wavelength
channel is transmitted, attenuated or completely blocked, is
preferably performed by the last LC element 61, which is pixilated,
while the steering is performed using alternate pixilated LC
crystals 62 and birefringent prismatic crystals 63, referred to
hereinafter as LC/prism pairs, which are arranged serially in the
beam paths. Each birefringent prismatic crystal deflects a beam
impinging thereon by an angle which is dependent on whether the
beam has s- or p-polarization, and the determination as to whether
the beam impinging on a certain pixel has s- or p-polarization can
be selected by applying the appropriate control voltage to the
preceding LC pixel for that channel. Since each prism preferably
selects one of two steering angles (assuming that the LC is driven
to generate polarization rotations of) 90.degree., then the number
of possible steering angles becomes 2.sup.n, where n is the number
of LC/prism pairs used in the router. Thus for three LC/prism
pairs, 8-way steering is possible. In FIG. 6A, a mirror 65 is shown
after the assembly to reflect the output beams back through the
router.
[0078] In the plan view of the steering focal plane module of FIG.
6A, the beam deflection angles are into or out of the plane of the
drawing, such that the differently directed beams are not
discernible. Reference is now made to FIG. 6B, which is a schematic
side view of the preferred transmissive LC/prism focal plane
steering array shown in plan view in FIG. 6A, showing the different
directions into which the module directs the input channel. Each of
the three LC/prism pairs 64 can steer the beam into one of two
different directions, depending on the beam polarization, such that
8 different steered directions are provided with the three stages
shown. Birefringent prisms 63 rather than slabs are used in order
to ensure that each of the birefracted beams is directed towards a
different angle, ensuring channel separation between ports.
Furthermore, each prism should preferably have a different wedge
angle, .theta..sub.1, .theta..sub.2, .theta..sub.3, to ensure
compete angular separation of the steered beams from each stage,
regardless of whether birefracted or not. The LC element 61 for
selecting the switching status is preferably disposed either before
or after (as shown in FIGS. 6A and 6B) the beam steering assembly,
but not within the beam steering module, in order to avoid
interference of the beam steering by the polarization changes
caused by the switching element, which would cause channel
cross-talk.
TABLE-US-00001 TABLE I Output after Output after Output after Input
1st LC 2nd LC 3rd LC P Beam to port 1 S S S Beam to port 2 S S
P
[0079] Reference is now made to Table I, which shows the
polarization states of outputs 1 and 2 of the preferred
transmissive embodiment of FIG. 6B, for a situation where the
switching LC 61 after the beam steering module is set to provide no
additional polarization change in any output beam, i.e., all the
beams are fully transmitted. For a p-polarization input beam, and
for the illustrated settings of the LC beam steering cells shown in
FIG. 6B, in which the p-polarization is deflected more than the
s-polarization beam, the output at port 1 has an s-polarization,
whereas that at port 2 has a p-polarization. Therefore, it is
apparent that since the beam steering module generates polarization
changes in the output signal polarizations, quite separately from
the polarization changes engendered by the switching process LC, a
low polarization dependent loss (pdl) grating must be used in these
embodiments, to ensure that the dispersive element can handle beams
of differing polarization. The outputs to the other ports 3 to 8 in
Table I can be similarly displayed.
TABLE-US-00002 TABLE II After: Before: Input LC1 LC2 LC3 LC3 LC2
LC1 Output P Beam to port 1 S S S MIRROR S S S P Beam to port 2 S S
P MIRROR P S S P
[0080] Reference is now made to Table II, which shows the
polarization states of outputs 1 and 2 of the preferred reflective
embodiment of FIG. 6A, again for a situation where the switching LC
after the beam steering module is set to provide no additional
polarization change in any output beam. For a p-polarization input
beam, and for the same settings of the LC beam steering cells as
those shown in FIG. 6B, the incident polarization at the mirror 65
for the beam destined for output port 1 is s-polarization, whereas
that destined for port 2 has a p-polarization. However, since the
beams are now reflected by the mirror, they return in a reverse
path back to the input of the beam steering module, which is now
the output of the beam steering module, and undergo the opposite
polarization changes in the return path to those that they
underwent in the incident path. As a result, each reflected output
beam has the same polarization as that of the incident beam, and
the beam steering module itself does not generate any polarization
changes in the output signal polarizations. Thus for a fully
transmitted signal, where the switching LC 61 does not introduce
any other polarization changes, a high efficiency grating can be
used to handle the like-polarized transmitted beams of all of the
channels.
[0081] Reference is now made to FIGS. 7A and 7B, which are
schematic views of two alternative and preferred transmissive beam
steering modules, similar to those illustrated in FIG. 6B, having
liquid crystal cells 70 with associated birefringent crystal
prisms, 71, 72, 73, each prism having a different wedge angle, and
a switching liquid crystal transmission element 74, but showing
how, in FIG. 7B, the orientation of the wedge of the various prisms
can be varied compared with that of FIG. 7A, without affecting
performance. In the example shown in FIG. 7B, one of the prisms 76
is aligned such that it deviates the beams in the opposite
direction to that of the other prisms. All that is required is that
the chosen deflection angles should provide clear beam separation
between output ports.
[0082] Reference is now made to FIGS. 8A and 8B, which are
schematic illustrations of the fiber interface and polarization
conversion module, according to a further preferred embodiment of
the present invention. FIG. 8A is a side view, with the input and
output fiber collimators 80 shown directing their beams at the
birefringent polarization converter 81, which is preferably shown
as a YVO.sub.4 crystal. In FIG. 8B is shown a front view of the
birefringent crystal 81, showing the array of beams exiting the
birefringent crystal, a pair from each collimator, with a half wave
plate 82 covering one side of the outputs of the birefringent
crystal in order to rotate the polarization of one set of these
output beams by 90.degree. such that both beams of any port have
the same polarization, as is known in the art.
[0083] Although the optical signals are schematically shown in FIG.
8A as being input or output from an array of single fiber
collimators, it is to be understood that any other methods known in
the art for inputting or outputting a plurality of signals can also
be used in the present invention. An important feature of the
input/output arrangement is that the optical channels be spaced as
close as possible, in order to provide the most compact router as
possible, with concomitant savings on component costs, and with
increased packaging density for the routers in a communication
system. Thus, it is possible to use a V-groove array to provide
closer packing, either with a microlens array, or with a single
lens for all of the inputs/outputs. Similarly, a waveguide input
device can be used, in which the fiber inputs/outputs are brought
closer together in a monolithic waveguide structure, such as can be
fabricated in a silicon substrate. Even closer spacings can be
achieved by this means.
[0084] Reference is now made to FIG. 9, which is a schematic side
view of the fiber interface input and polarization conversion
module 90, 91, shown in FIG. 8A, but including an inverse telescope
92 for demagnifying the height of the array of input beams. This
demagnification is in the plane generally orthogonal to that in
which the lateral beam expansion takes place. Such demagnification
is preferable used because the physical size of the input fiber
collimators makes their spacing significantly larger than that
required in the other optics modules of the router. The original
height 93 of the beam array exiting and entering the collimator
array 90 is reduced to a beam height 94 of significantly smaller
dimensions for directing into the remainder of the router
components, thus enabling the achievement of a more compact router
geometry than would be achieved without such beam
demagnification.
[0085] Reference is now made to FIG. 10, which is a schematic side
view of an alternative configuration of the fiber interface input
module shown in FIG. 9. In this preferred embodiment, the inverse
telescope 101 for demagnifying the height 103 of the beam array, is
positioned before the input to the polarization conversion
birefringent crystal 104, such that this crystal too, along with
the rest of the router components, can have reduced height
commensurate with the reduced height 102 of the demagnified
beam.
[0086] Reference is now made to FIGS. 11A and 11B, which illustrate
a schematic wavelength selective Add/Drop router module,
constructed and operative according to a further preferred
embodiment of the present invention. FIG. 11A is a block diagram of
the functionality, while FIG. 11B shows the implementation of the
beam steering in such a system using MEMS mirrors. This Add/Drop
router uses beam steering modules 110 such as those described in
any of the various embodiments previously described in the present
application. A circulator (not shown in FIG. 11B) is required at
the main input/output port, in order to separate the input from the
output signals in reflective embodiments. In such a case, the
difference between an Add port and a Drop port is essentially one
of nomenclature only, since the optical path between the
input/output port and each of the Add or Drop ports is determined
only by the beam switching and the beam steering commands, and
these can be selected as desired.
[0087] This feature is illustrated schematically in FIG. 12, which
shows the different steering angles generated by a MEMS mirror
array 120, directing the beam between the input/output port and any
of the other ports. The decision as to whether any port is called
an Add or a Drop port is determined only by the direction of the
signal being transmitted, whether into or out of the selected
port.
[0088] It is also feasible to construct an Add/Drop router,
according to a further preferred embodiment of the present
invention, without the need for a circulator, using separate input
and output ports, each of which utilizes a separate steered angle
in the beam steering module. However, in this case, the control and
programming of the beam steering array is significantly more
complex, since the beam steering module then has to be programmed
so that each port can be connected to any of the other ports. This
also makes the accuracy of aiming of the MEMS mirrors significantly
more critical, to ensure providing sufficient angular steering
resolution for the increased number of steering angles
required.
[0089] In the previously described MEMS embodiments of the beam
steering module, the MEMS devices have been mirror arrays in a
reflective embodiment of the router, with the output beams
traversing essentially similar paths to those of the input beams,
and passing through the same components as were used for the input
beams.
[0090] Reference is now made to FIG. 13, which illustrates a
further preferred embodiment of the present invention, in which
MEMS devices, based on reflection from mirrors, are nevertheless
used in a transmissive embodiment of the router, in the sense that
the input and output sections of the router are spatially distinct.
The WSS shown in FIG. 13 comprises an input section, similar to
that shown in the embodiment of FIG. 4A, with a fiber collimator
followed by a birefringent walk-off crystal 131 with a half wave
plate 139 over part of the output face, a beam expansion prism 132,
a dispersive element 133, and a focusing lens 134 for focusing the
beams onto a beam switching and steering module preferably
comprising an LC device 136 and a MEMS array 130. However, unlike
the embodiment of FIG. 4A, the steering module of FIG. 13 is
aligned at an angle significantly different from normal incidence,
such that the beams reflected from the MEMS mirrors are diverted in
a direction away from the input optics path, and towards a
completely separate set of optics components 135, generally
equivalent to those used on the input side, and acting as an output
optics system. As previously, the individual MEMS mirrors 130 are
tilted at small angles around the average alignment direction, in
order to steer individual beams along their desired path and to the
on the output section 137 of the router. The output optics system
can be either horizontally or vertically displaced from the input
plane, depending on the geometrical configuration preferred, and on
the feasibility of that geometrical configuration.
[0091] Reference is now made to FIG. 14, which illustrates a
further preferred embodiment of the beam steering module for use in
the reflective embodiments of the present invention. According to
this embodiment, the beam steering can be generated by use of a
phased array liquid crystal-on-silicon (LCOS) device, with a phased
linear array for each wavelength component. The linear phased array
is then programmed to direct the beam to the direction desired for
that wavelength according to the phase shifts applied to the
various pixels in the phased linear array for each wavelength. A
complete 2-dimensional LCOS array is then able to direct all of the
wavelength channels of the device.
[0092] In FIG. 14, there is shown a schematic representation of a
single wavelength channel of an LCOS beam steering assembly. The
assembly preferably comprises a pixilated liquid crystal
polarization rotation array 141, for selecting the desired
transmissive, blocked or attenuated state of each wavelength
channel, an optional linear polarizing element 142 to increase the
extinction ratio of the polarization selection combination in the
system, and an LCOS array 143 preferably comprising a thin layer of
liquid crystal material disposed on top of a pixilated CMOS driving
array, for reflectively steering the beam from the specific
wavelength pixel shown in FIG. 14 back through that pixel, at a
selected steered angle. The steered angle is dependent on the field
applied to the LC elements of the phased array by the CMOS pixels
in the LCOS array, each field generating a different phase shift in
the light passing through the LC layer of the LCOS array. The
arrangement of successive phase shifts in the array defines the
steered angle, as is known in phased array technology.
[0093] According to another preferred embodiment of the LCOS beam
steering phased array, it is possible to forgo the need for the
separate pixilated liquid crystal polarization rotation array 141,
and to perform both the attenuation and steering functions by means
of the LC layer on the LCOS array. The desired attenuation can be
achieved either by adjusting the reflected phase pattern to be less
than optimal, such that part of the incident beam is not specularly
reflected from the LCOS array, and the switched beam is thus
attenuated as well as being steered, or by using optimal efficiency
phased array steering, but adjusting the steering direction
slightly so that the output beam does not fully overlap the output
port, so that only part of the output beam is coupled out, as shown
in the embodiments of FIGS. 5C and 5D. This embodiment leads to a
simple beam steering module, although the control thereof is more
complex.
[0094] In the preferred embodiment of FIG. 14, the beam steering
module is shown with one input port and 3 output ports, though, as
previously explained in relation to the MEMS embodiments of the
present invention, this division is nominal, and any port can be
used for any function, whether input or output. The beam steering
arrays for the other wavelength channels to be handled by the WSS
are disposed in the direction perpendicular to the plane of the
drawing of FIG. 14.
[0095] The steering angles achievable with such an LCOS phased
array are very small, generally of the order of a very few tenths
of a degree. In FIG. 14, the deflection angles have been
exaggerated, in order to render the operation of this embodiment
visible. It is therefore generally difficult to use an LCOS beam
steering array in practice without an auxiliary beam deflecting
component 144, whose function is to increase the beam deflection
angle so that the deflected beams can be resolved, and the beam
steering array can be practically used. Such a deflection
amplifying device can preferably be constructed using a diffractive
optical element (DOE) or a holographic element, or a sequential
series of reflecting surfaces, each successive reflector doubling
the deflection angle achieved, or a divergent prism arrangement,
though it is to be understood that the invention is not meant to be
limited to these solutions.
[0096] Any of the above described embodiments of the WSS of the
present invention can include a number of auxiliary functions which
increase the usefulness of the device in practical systems. Channel
power monitoring can be performed by splitting off a small
percentage of the beam power, preferably at the output or drop
ports, and this power coupled out can be directed onto an array of
detectors which are used for determining the output power in each
channel. Additionally, in those embodiments using a flat array of
fibers as channel inputs, the signals can be input to the WSS at
predefined lateral positions by using a V-groove input block, as is
known in the art. Furthermore, a silicon waveguide array can be
used at the input, with channels spaced as close as 9 microns from
each other, thus reducing the size of the device. Use of a
microlens array for focusing the input beams can then be
advantageous.
[0097] Reference is now made to FIG. 15, which schematically
illustrates a plan view 150 of one exemplary implementation of an
optical switch according to an embodiment of the present invention.
In this implementation any number of input signals each received at
a different input port, may be directed towards any of the output
ports. It is generally assumed that the input and output signals
may include a WDM signal or a multi-pole optical beam. The optical
switch accepts a plurality of input signals arriving at different
input ports and converts them into free space collimated beams for
focusing, polarization, and spatial manipulation.
[0098] For clarity, only one input signal 151 and one input port
152 of the optical switch are shown and labeled in the plan view
where the input signal 151 is shown coupled to the optical switch
via a fiber interface 152. A polarization selection module 153 with
a half wave plate 154 covering part of the polarization selection
module, adjusts the polarization state of the free space beam. The
polarization selection module generates a pair of beams 173
(hereinafter input beam pair) with substantially the same
predefined polarization state in a predetermined plane (shown as
vertical lines 155). Regardless of the polarization state of the
input beam, the input beam pair has substantially similar
predetermined polarization state in the optical switch. A
predetermined polarization state so applied allows controlled
attenuation of the beam pairs at the output ports, by applying a
predetermined polarization rotation. A focusing element 156 focuses
the input beam pair onto a pixilated beam steering module 157.
[0099] The beam steering module 157 comprise two parts--a pixilated
one-dimensional array of beam steering elements 159 and a pixilated
one-dimensional array of polarization control elements 158. In this
exemplary configuration, the beam steering array 159 and the
polarization control array 158 are located in two different optical
paths. However, it is not necessary to be so in other
configurations that will be apparent to those skilled in the art.
Each beam steering array comprises a plurality of pixels
(hereinafter beam steering pixel) that are independently tiltable
along a single rotation axis in response to a control signal. Each
beam steering pixel deflects the incident input beam pair
preferably in a plane perpendicular to the plane of the drawing and
towards a specific output port 162 selected in accordance with the
tilt applied to the beam steering pixel. In this manner, any input
beam pair can be directed to a desired one of the output ports.
[0100] The deflected beam pair 174 impinges upon a pixel of the
polarization control array (hereinafter polarization control
pixel). Each polarization control pixel in response to a respective
control voltage proportionally rotates the polarization of the
input beam pair impinging thereupon. The polarization control array
applies rotation to the input beam pair in accordance with the
predetermined polarization state set by the polarization selection
module. The deflected beam pair 174 after undergoing polarization
rotation is collimated by a second focusing element 166. The second
focusing element 166 directs the deflected beam pair through a
polarization conversion module 163.
[0101] A half-wave plate 164 covers a part of the polarization
conversion module on the side where the deflected beam pair is
incident on the module. The polarization conversion module is
similar to the polarization selection module configured in reverse
and functions in a complementary mode as compared to the
polarization selection module. The polarization conversion module
at the output port is set to receive signal having a predetermined
polarization state. In one exemplary embodiment, the polarization
conversion module is set to allow signals having the same
polarization state as the input beam pair, to pass and generate an
output signal. Deflected beam pairs arriving at the polarization
conversion module with their polarization states rotated from the
polarization state of the input beam pair, are not allowed to pass.
Accordingly, no output signal is generated thereby, attenuating the
deflected beam pair at the output port. The degree of attenuation
is proportional to the amount of polarization rotation applied in
the polarization control array.
[0102] For example, if a polarization control pixel of the array
does not apply any rotation to the deflected beam pair (total
rotation equals 0.degree.), the deflected beam pair arrives at the
polarization conversion module of an output port in the same
polarization state as during its transmittal, and thereby passes to
the selected output port without attenuation. In this manner, the
input beam pair deflected by the beam steering device towards a
selected output port, exits out of the selected port as an output
signal. On the other hand, if a polarization control pixel applies
a 90.degree. rotation the beam pair arrives at the polarization
conversion module of the output port in a polarization state that
is different from the polarization state of the input beam pair
during its transmittal, and thereby does not pass through the
polarization conversion module and experiences a full attenuation
set by the extinction ratio of the system. Any amount of
attenuation may be applied in one or more steps. For example, in
another exemplary arrangement, the polarization control array may
be disposed in a path such that the beam pair passes the
polarization control pixels twice, once each before and after being
deflected by the beam steering pixel. In this configuration, the
polarization control pixel rotates the beam by 45.degree. in each
pass to achieve full attenuation. Intermediate configurations may
provide any level of attenuation desired. For example, in other
exemplary embodiment two polarization control arrays may be
disposed in the beam pair path such that each polarization control
array applies a predetermined amount of polarization rotation in
each pass to achieve full attenuation. These and other arrangements
will be apparent to those skilled in the art within the scope of
the disclosure of the present invention.
[0103] While only one polarization conversion module is shown in
FIG. 15, there is an array of similar elements, each one
corresponding to an output port. The polarization conversion module
combines each deflected beam pair into an output beam. The
polarization conversion module is similar in structure as the
polarization selector module except it is configured to operate in
a complementary mode. The polarization conversion module 163
combines the deflected and polarization rotated beam pairs 174 into
an output beam and, directs the output beam to a selected output
port 161 via an output collimator 162. As described above, the
selection of the output port is determined by the amount of
deflection applied in the beam steering module.
[0104] Reference is now made to FIG. 16, where 200 is a schematic
isometric view of an m.times.n switch including a complete array of
input and output ports 202 (1 . . . m) and 212 (1 . . . n),
respectively. In this implementation, any number of input signals,
each received at a different input port, may be directed towards
any of the output ports. In the following description, each element
shown in FIG. 16 is labeled and described only once for clarity.
However, following description is pertinent to each of the
multiplicity of identical elements and functions substantially in
the same manner as their similar counterparts described in
reference with FIG. 15.
[0105] Each input signal 201 is coupled via a fiber interface 202
of the array of input fiber interfaces 202 (1 . . . m). The fiber
interface 202 collimates the input signal and converts said input
signal to a free space optical beam. Each input signal converted
into a free space beam, passes through a polarization selection
module 203 with a half wave plate 204 covering a portion of it, and
generates a pair of beams 223 (hereinafter input beam pair) having
a predetermined polarization state irrespective of the polarization
state of the input free space beam. The direction of polarization
is indicated by vertical lines 205. Each input beam pair is focused
by a focusing element 206, on to a pixilated beam steering module
207 placed at the focal plane of the focusing element.
[0106] The pixilated beam steering module 207 comprises a pixilated
beam steering array 209 and a corresponding pixilated, polarization
control array 208. In the exemplary configuration shown in FIG. 16,
the beam steering array 209 and the polarization control array 208,
respectively, are disposed in two different optical paths. However,
it is not necessary to be so in other configurations that will be
apparent to those skilled in the art. Each beam steering array
comprises a plurality of pixels (hereinafter beam steering pixel)
that are independently tiltable in accordance with a control
signal. Each beam steering pixel therefore deflects the incident
input beam pair preferably in a plane perpendicular to the plane of
the drawing, and towards a specific output port 212 of the
plurality of output ports (212 (1 . . . n)), selected in accordance
with the tilt applied to the beam steering pixel.
[0107] The pixels of the polarization control array 208
(hereinafter polarization control pixel) selectively applies a
controlled polarization rotation to each deflected beam pair 224.
Each polarization control pixel in response to a respective control
voltage, applies a predetermined amount of polarization to each of
the deflected beam pairs impinging thereupon, such that the
deflected beam pairs arrive at respective output ports they are
destined for in the same predetermined polarization state as the
input beam pair (total rotation 0.degree.). Alternatively, the
polarization control pixel applies a polarization rotation by a
finite angle (e.g., 90.degree.) for the deflected beam pairs that
are targeted for attenuation so that they do not arrive at output
ports they are not supposed to reach.
[0108] The deflected beam pair 224 is collimated by a second
focusing element 216. The second focusing element directs the
deflected beam pair back through a polarization conversion module
213 which is similar to the polarization selection module in
reverse, including a half-wave plate 214 covering a part of the
polarization conversion module on the side where the deflected beam
pair is incident on the module. The polarization conversion module
combines the deflected beam pair arriving with the same
predetermined polarization state as the input beam pair, into an
output beam and directs the output beam to a selected output port
via an output collimator 212. The selection of the output port is
determined by the amount of deflection and the polarization
rotation applied in the beam steering module.
[0109] It should be recognized that the deflected beam pairs that
arrive at an output port with a polarization rotation are not
combined into an output beam and are thereby attenuated by an
amount proportional to the applied polarization rotation. In this
manner, an output beam that is not destined to reach any particular
output port is blocked partially or completely, without interfering
with the other output signals reaching that particular output port
in a `hitless` operation.
[0110] In the exemplary embodiments shown in FIGS. 15 and 16, the
polarization selection module may be a birefringent walk-off
crystal such as, Yittrium Ortho Vanadate crystal (YVO.sub.4).
However, other polarization components and methods well known in
the art for introducing polarization diversity may be used without
deviating from the basic principles of the current invention.
[0111] In FIG. 15, the first and second focusing elements are shown
as lenses 156 and 166 only for illustrative purposes and are not
limited only to lenses. Any focusing element capable of spatially
separating different input beam pairs and subsequently focusing
each one of the input beam pairs onto separate single pixel of the
beam steering module may work equally well. Other focusing elements
known in the art for example, a reflective component may equally be
effective for this purpose. Accordingly, the beam steering module
may be placed differently in the arrangement of the optical
switch.
[0112] In another embodiment, the focusing elements 206 and 216
shown in FIG. 16 may be a single lens covering all the internal
optical paths within the optical switch or a lens array, or a
combination thereof. In embodiments where a lens array is used,
individual lenses of the lens array may be disposed in each
individual input and output optical signal paths. Such a
configuration will be described later in reference with FIGS. 17A
and 17B.
[0113] The beam steering module shown in FIGS. 15 and 16 may be
implemented by a one-dimensional array of micro-electromechanical
system (MEMS) mirrors where individual MEMS mirrors have a single
rotation axis, and the polarization control array may comprise a
liquid crystal (LC) array. Each pixel of the MEMS mirror array is
controlled independently to apply a desired deflection and each
pixel of the LC array is independently controlled to apply a
desired polarization rotation to the beam pair passing
therethrough. Each polarization combiner at the respective output
ports either allows or blocks the deflected beam pair from reaching
the output ports in a `hitless` operation depending upon the
polarization state of the deflected beam pair.
[0114] For the purpose of discussion, a `hitless` operation is
defined as a switching operation where the input beam to be
switched to a selected output port is scanned by a corresponding
MEMS mirror but do generate spurious output signals at any of the
non-selected output ports. The beam steering module constructed
according to the principles of the current invention achieves a
`hitless` operation by applying in a corresponding polarization
control pixel, a predetermined polarization rotation to the beam
pair while the beam pair is scanned by the corresponding MEMS
mirror. After the beam pair has been scanned such that it has been
switched to the selected output port, the predetermined
polarization rotation by the corresponding polarization control
pixel is no longer applied.
[0115] FIGS. 17A and 17B schematically illustrate different
implementations of the multi-pole optical signal switch of FIGS. 15
and 16, in which the input and output fiber ports are aligned in
one contiguous array. The two devices 300 and 310 shown in FIGS.
17A and 17B, respectively, differ from the devices shown in FIGS.
15 and 16 in the arrangement of input and output ports. As shown in
FIGS. 17A and 17B, `m` input and `n` output fiber ports are aligned
in one contiguous array 320. More specifically, the input and
output ports may be arranged alternately, or with all the input
ports at one end and all the output ports at the other end of the
array as shown in FIGS. 17A and 17B. Advantageously, construction
of the device in this implementation is very simple. All of the
optical switching operations take place in a single plane, namely
the plane connecting the input fiber array 320 with the
one-dimensional pixilated beam steering array 309. In the exemplary
embodiments schematically shown in FIGS. 17A and 17B, the focusing
element is a lens array 322. In this implementation, each pixel of
the one-dimensional beam steering array has a larger angular
deflection than the embodiment shown in FIG. 16. An optional linear
polarizing element 325 may be disposed for example before the beam
steering module as shown in FIGS. 17A and 17B.
[0116] Additional embodiments of the present invention are shown
schematically in FIGS. 18A and 18B. In these embodiments of optical
switch 400, an input array of fibers 401 and an array of output
fibers collectively referred as 415, are aligned essentially
perpendicular to each other. In the illustrative examples shown in
FIGS. 18A and 18B, focusing elements are not shown for clarity. The
output ports shown in FIG. 18A are arranged in a one-dimensional
array. In an alternative embodiment, shown in FIG. 18B, the output
ports are arranged in a two-dimensional array. As shown in FIGS.
18A and 18B, two one-dimensional beam steering modules 407 and 417
are arranged perpendicular to each other in two different planes.
In particular, the beam steering module 407 steers the input beam
pair to the plane of the second beam steering module 417 which, in
turn steers the deflected beam pair in a plane perpendicular to
that of the incident beams and to a desired output port in
accordance with the control signals applied to individual pixels of
the beam steering arrays 407 and 417. In an exemplary
implementation, the beam steering modules include a one-dimensional
MEMS mirror arrays with single axis of rotation, for beam steering,
in combination with a one-dimensional LC arrays for polarization
control of the individual output beams. One advantage of this
particular arrangement is that the angular range required of each
pixel of the beam steering array is reduced as compared to the
arrangements shown in FIGS. 15-17. While the arrangement shown in
FIGS. 18A and 18B requires two one-dimensional beam steering
modules, it may still be cost-effective relative to the
arrangements shown in FIGS. 15-17 because the control system
required for two one-dimensional beam steering array may be less
than the cost of implementing a more complex control system for a
two-dimensional MEMS array.
[0117] In all of the embodiments described above, the image
attenuation is performed by rotating the polarization of the beams
using a pixilated LC array. It is also possible to generate beam
attenuation by means of the phase control of the beam, in which
case there is no need for polarization diversity and its related
components. In addition, it is possible to use a rotator together
with a polarizer in order to convert the input beams to circular
polarization. This will entail a 3 dB power loss in passage through
the device, but will eliminate the effects of polarization
dispersion loss in passage through the LC array pixel. These and
other alternative implementations within the principles of current
invention should be apparent to those skilled in the art.
[0118] FIG. 19 shows an end view 500 of a polarization selection
module of FIGS. 15 and 16, with its half-wave plate covering one
part of the output port. A similar device operated in reverse, may
also be used as the polarization conversion module. The
polarization selection module in this exemplary embodiment is a
birefringent walk-off crystal with a half-wave plate 504 covering
one part of the output port. Each free space input beam arranged in
a vertical stack within the optical switch, traversing the
polarization selection module, is split into two beams 523 and 533
that form an input beam pair having substantially similar
well-defined polarization states irrespective of the initial
polarization state of the input beam.
[0119] In a multi-port router, steering an input signal to a
desired output port may require the requested signal to be steered
across other output ports. More specifically, referring back to
FIG. 17B, when an input signal 301(1) from an input port 302(1) is
steered between two output ports, 312(1) and 312(3), the steered
beam would momentarily cross the output port 312(2) placed in
between. As a result, an undesirable spurious signal will
momentarily appear at the output port 312(2). One important aspect
of the present invention is to provide a `hitless` beam steering.
In a `hitless` beam steering operation, the input beam would bypass
the port 312(2) that is not a selected output port without any
spurious signal appearing therethrough.
[0120] FIG. 20 illustrates schematically a hitless beam steering
configuration according to one or more embodiments of the present
invention. In an exemplary embodiment, the beam steering module may
be implemented by a combination of a pixilated one-dimensional MEMS
mirror array single axis steering and a pixilated LC array. In this
configuration, a pixel of the MEMS mirror array primarily deflects
the input beam incident thereupon, towards a desired output port,
and a corresponding pixel of the LC array rotates the polarization
of the input beam while the input beam is being deflected so that
the input beam is attenuated when it crosses the other output
ports. The combination of the LC array and the polarization
conversion module functions as a switching shutter.
[0121] For example, in the configuration shown in the view 603, the
optical switched beam 611 is steered directly to the destination
port 621 and the path over the other output ports is blocked. The
blocked path represented as 640, is achieved by applying an
appropriate amount of control settings to the LC array pixel
associated with the spatial channels over the output ports such
that the transmission is blocked (in other words, beam 611 is
completely attenuated) while the beam passes over the output ports
other than 621. Once the optical switching process is completed,
the transmission to the other output port may be unblocked and the
optical switch can operate as programmed. Thus, the operation of
the optical switch built according to the principles outlined above
automatically prevents spurious signals to reach the ports other
than the destination port.
[0122] One aspect of the beam steering module comprising the
one-dimensional beam steering array in combination with the
pixilated LC array lies in the attenuation control function of the
LC array pixel. An advantage of this aspect is that the output beam
624 may be completely coupled to the destination port 621 and any
degree of attenuation may be controllably achieved by applying a
polarization rotation in the LC array so as to generate the output
signal with an attenuation. Therefore, distortion to the pass band
shape may be significantly reduced or eliminated completely.
[0123] It is appreciated by persons skilled in the art that the
present invention is not limited by what has been particularly
shown and described hereinabove. Rather the scope of the present
invention includes both combinations and sub combinations of
various features described hereinabove as well as variations and
modifications thereto which would occur to a person of skill in the
art upon reading the above description and which are not in the
prior art.
* * * * *